Opamp offset voltage problem

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Guest

Posted: Fri Feb 02, 2007 2:36 am

Hi! I want to measure the offset voltage of an opamp. First, I
configure the opamp in an unity gain buffer and measure the output.
The offset voltage is 64.16uV. Then, I applied this voltage to the
non-
inverting input of the opamp. When I simulate it again, instead of
getting 0V at the output, I get 464.29mV. Is the setup correct? Is the
offset voltage of 64.16uV correct? Looks a bit small for an opamp.

Thanks.
Amy

* Schematics Netlist *

I_I1 0 $N_0001 DC 100u
M_M1 $N_0002 $N_0002 $N_0003 $N_0003 Mbreakp
+ L=1u
+ W=50u
M_M2 $N_0004 $N_0002 $N_0003 $N_0003 Mbreakp
+ L=1u
+ W=50u
M_M6 $N_0001 $N_0001 $N_0005 $N_0005 MbreakN-X2
+ L=1u
+ W=10u
V_V1 $N_0003 0 5V
V_V2 0 $N_0005 5V
R_R1 $N_0007 $N_0006 12.245k
M_M8 $N_0006 $N_0004 $N_0003 $N_0003 MbreakP
+ L=1u
+ W=100u
M_M14 $N_0003 $N_0006 VOUT VOUT MbreakN-X7
+ L=1u
+ W=10u
M_M7 $N_0007 $N_0001 $N_0005 $N_0005 MbreakN-X4
+ L=1u
+ W=20u
M_M5 $N_0008 $N_0001 $N_0005 $N_0005 MbreakN-X3
+ L=1u
+ W=20u
M_M15 $N_0005 $N_0007 VOUT VOUT MbreakP
+ L=1u
+ W=25u
M_M4 $N_0004 $N_0009 $N_0008 $N_0008 MbreakN
+ L=1u
+ W=20u
M_M3 $N_0002 $N_0010 $N_0008 $N_0008 MbreakN
+ L=1u
+ W=20u
V_V8 $N_0010 0 0V
V_V9 $N_0009 $N_0010 -64.16u

kell

Posted: Sun Feb 04, 2007 11:09 am

Guest

On Feb 1, 10:36 pm, amy_burton2...@yahoo.com wrote:

Quote:

Hi! I want to measure the offset voltage of an opamp. First, I
configure the opamp in an unity gain buffer and measure the output.
Here's another way that might be more reliable.

To measure offset:
ground the non-inverting input; run a pot from the op-amp output to
ground; connect the pot wiper to the inverting input. Measure the
output voltage. Calculate offset at the input as output voltage times
1+(R2/R1), where R1 is the grounded side of the pot and R2 is the side
connected to the op amp output.

Quote:

The offset voltage is 64.16uV. Then, I applied this voltage to the
non-
inverting input of the opamp. When I simulate it again, instead of
getting 0V at the output, I get 464.29mV.
Is the op amp open loop? I don't think that's how you null them out.

There are nulling cicuits here:
http://www.google.com/search?hl=en&q=op+amp+circuit+collection

kell

Posted: Sun Feb 04, 2007 11:12 am

Guest

On Feb 4, 7:09 am, "kell" <kellrobin...@billburg.com> wrote:

Quote:

On Feb 1, 10:36 pm, amy_burton2...@yahoo.com wrote:> Hi! I want to measure the offset voltage of an opamp. First, I
configure the opamp in an unity gain buffer and measure the output.

Here's another way that might be more reliable.
To measure offset:
ground the non-inverting input; run a pot from the op-amp output to
ground; connect the pot wiper to the inverting input. Measure the
output voltage. Calculate offset at the input as output voltage times
1+(R2/R1), where R1 is the grounded side of the pot and R2 is the side
connected to the op amp output.

D'oh, or just use a couple of resistors.

Jamie

Posted: Sun Feb 04, 2007 12:43 pm

Guest

kell wrote:

Quote:

On Feb 1, 10:36 pm, amy_burton2...@yahoo.com wrote:

Hi! I want to measure the offset voltage of an opamp. First, I
configure the opamp in an unity gain buffer and measure the output.

Here's another way that might be more reliable.
To measure offset:
ground the non-inverting input; run a pot from the op-amp output to
ground; connect the pot wiper to the inverting input. Measure the
output voltage. Calculate offset at the input as output voltage times
1+(R2/R1), where R1 is the grounded side of the pot and R2 is the side
connected to the op amp output.

The offset voltage is 64.16uV. Then, I applied this voltage to the
non-
inverting input of the opamp. When I simulate it again, instead of
getting 0V at the output, I get 464.29mV.

Is the op amp open loop? I don't think that's how you null them out.
There are nulling cicuits here:
http://www.google.com/search?hl=en&q=op+amp+circuit+collection

you know, back in the old days! i think it is the 741? pins 1 and 8

gave you nulling inputs you could put a pot on with the wiper to
- to fix that little problem. :)

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

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